The invention relates to an apparatus for the continuous or quasi-continuous analysis of test substances, which presents a closed system in that all external intervention can be avoided, provided that it is not necessitated by failure indications. If indications of failure do not throw the measured values produced into doubt, the measured values can be considered to be reliable. Routine operations (calibration, condensate delivery from a test gas condenser, blow-through of an extraction probe and the like) are carried out automatically. The test substances can be present in a gaseous or liquid phase.
Process analysis measuring devices consist of an extraction device, a processing chain and at least one analysis instrument, the individual parts being designed in different ways depending on the object set. For example, the extraction device and processing chain are generally very simple in design for monitoring environmental air whereas they are usually of quite a complex configuration for measuring emission. The functioning of the analysis instrument is normally dependent upon the correct functioning of the extraction device and the processing chain. In process analysis measuring devices it is important for the measured values to be readily available and reliable. Up until now, daily checks of operation have generally been carried out by skilled workers to achieve sufficient availability and reliability. The analysis instruments are calibrated regularly to maintain the accuracy of measurement. This method of preventive maintenance has the disadvantage, in addition to the associated high cost, that disturbances occurring between two checks of operation can remain unnoticed. This applies, in particular, to those disturbances which do not affect the measurement value (for example, insufficient permeation of test gas due to a blockage).
In the past, it has been proposed that the availability of analysis measurements be increased while simultaneously reducing the operating costs by using process computers. In this arrangement, the conditions of status sensors should be transmitted to a measuring device and the measured value to a central computer which examines the plausibility of the measured value and checks the operation of the measuring device using the status indications. The computer should be able to intervene in the measuring device and, for example, effect calibration by remote control.
This design is only suitable for extensive measuring networks due to the associated cost (computer software, remote control system). Moreover, in principle, both the computer and remote control system threaten the availability of the measured values.
Due to this fact, this design has hitherto only been adopted in environmental protection measuring networks as far as we know.
There are some analysis instruments which indicate deviations from the desired operating condition by means of status displays. There are also auxiliary devices which carry out calibration according to a fixed time program or are initiated from outside. In this case, the analysis instrument is charged with null gas or sensitivity gas. In certain conventional commercial instruments, the measured values determined are stored and the following measured values are corrected electronically on the basis of the stored calibration values. The disadvantage of this method is that the measured value is transposed into a correction circuit. If this is absent, the measured value is not available. Although a failure indication is produced in these automatic balancing devices when the calibration values have drifted from a certain range, undetectable operating conditions in which incorrect values are stored can exist. In another instrument, which is marketed in conjunction with IR analysis devices, the zero point and sensitivity are adjusted on the analysis instrument by means of motor potentiometers. Although the measured value is not hereby corrected, defective adjustments are not identified by failure indications.
The apparatuses just described have the disadvantage that only parts of the measuring device are monitored (status displays on analysis instruments) or only parts of the routine operations are automated (automatic equalization devices), the availability of the latter being impaired in principle by transposition of the measured value and/or irregular calibrations not being identified externally during routine operations.